A functional tear film is necessary for ocular health and vision. Tear quality, quantity, lid closure and blink are all important in maintaining a lubricated ocular surface. The ocular surface is a complex and dynamic environment, disturbance of which can be multi-factorial.
The tear film is formed of three layers. In contact with the corneal epithelium is the innermost mucin layer, one function of which is its action as a wetting agent. The middle layer is aqueous and is derived principally from the lacrimal glands. It constitutes the bulk of the tear film. The most superficial layer is oily and is derived from the meibomian glands in the eyelid. Meibomian gland drainage is a vital component to the healthy tear film. There are approximately forty of these small structures in each upper lid and thirty in each lower lid.
During the blinking process, probably at the point of lid closure, lipid is released from the meibomian glands onto the tear film layer. The principal function of this oily layer is to prevent tear evaporation. (See Reference 1). Additionally it, enhances tear stability, prevents tear spillover, sebum contamination and helps seal the lids at night. A smooth lipid layer also provides an ideal surface for refraction and the creation of a sharp retinal image. (See Reference 2).
Meibomian gland dysfunction [MGD] may occur spontaneously or be part of an oculocutaneous disorder such as blepharitis, atopic keratoconjunctivitis or generalized sebaceous gland dysfunction. MGD is a major cause of ocular surface abnormalities and ocular discomfort. (See Reference 3).
MGD and consequent oily tear film disturbance is a frequent clinical problem. One of the typical features of MGD is meibomian gland orifice obstruction. One manifestation of obstruction is capping of gland orifices where the surface of the orifice becomes covered with a layer which can be pierced with a needle and allowed to drain freely. (See Reference 4). It is possible that the surface of the capped gland is covered with an oxidized and saturated and therefore solid lipid covering. An adequate release of the oily tear film depends on the physical drainage of the oily layer. The melting point range of meibomian oil is reported as 19.5 to 32.9° C. (See Reference 5). Release of lipid is also related to the lipid constitution and the interplay of the musculature of the eye lids and meibomian orifices which, on lid closure, physically expel the lipid onto the tear film.
Animal studies have shown that experimental obstruction of these meibomian glands leads to ocular surface abnormalities found in keratoconjunctivitis sicca [dry eyes]. (See Reference 6). Studies in human volunteers have demonstrated an increase in the tear film lipid layer following a program of manual expression. (See Reference 7). Contact lens patients with MGD who undergo Meibomian therapy of lid scrubs and massage have been shown to have an objective increase in tear quality. (See Reference 8). Tear stability is increased following expression of the meibomian glands in healthy volunteers. (See Reference 9). Therefore an increased lipid thickness leads to improved tear quality and patient comfort. An important factor in the release of lipid is the ambient temperature level as, at higher temperatures, lipids are more liquid. Therefore, if the temperature of the lids can be increased and, if necessary, mechanical manipulation of this soft tissue induced, then the balance can be tipped from blockage to drainage.
Current advice to patients with obstructive MGD is to place hot flannels over the eyelids and massage the lids to increase the drainage. However patients find this is inconvenient and poor patient compliance is therefore an obstacle to effective therapy.
Patients with aqueous deficient dry eye often have associated lipid layer abnormalities and would benefit from an improvement in their tear lipid layer. Dry eye patients would also benefit from an environment that minimizes evaporative loss.
It is accordingly an object of the present invention to provide a therapeutic eye and eyelid cover that can be used to enhance tear quality.
It is a further object of the present invention to provide a therapeutic eye and eyelid cover that can be used to benefit patients having the above conditions.